1. Field of the Invention
This invention pertains to a catalytic composition for use in a hydrocracking process to maximize middle distillate yields for hydrocarbons boiling above about 600.degree. F. In particular, this invention concerns a hydrocracking and isomerization process to selectively produce low pour point middle distillate hydrocarbons from a hydrocarbonaceous feed boiling above about 600.degree. F. by contacting the feedstock with a catalyst comprising a specific class of silicoaluminophosphate molecular sieves and a hydrogenation component.
2. Description of the Prior Art
Hydrocracking, used either in a one-step process or in a multistep process coupled with hydrodenitrofication and/or hydrodesulfurization steps, has been used extensively to upgrade poor-quality feeds and to produce middle distillate materials. Over the years, much development work has been done into finding improved cracking conditions and catalysts. Tests have used catalysts containing only amorphous materials and catalysts containing zeolites composited with amorphous materials.
Large pore size zeolites such as zeolites X and Y are presently considered the most active hydrocracking catalysts. However, high activity is not the only essential characteristic of midbarrel cracking in catalysts. Midbarrel selectivity, namely, the percentage of total conversion accounted for by products boiling within the midbarrel range, from about 300.degree. F. to about 725.degree. F., is also important. In fact, as noted in U.S. Pat. No. 3,853,742, Ward, Dec. 10, 1974, many commercial midbarrel hydrocracking processes do not use zeolitic catalysts due to their relatively low midbarrel selectivity.
Also, middle distillates conventionally serve as fuels such as diesel oils, furnace oils, and the like. For convenience, in the handling and in the use of these middle distillates, it is desirable for the pour point to be as low as practical consistent with the temperatures to which they may be exposed. Specifications for these products often include a requirement that the pour point may not exceed a certain maximum value. In some instances it is necessary to subject these distillate fuels to additional processing whose principle purpose is to reduce the pour point of the feed stream. Pour point can also be lowered by lowering the distillate end point, but this reduces yield.
As noted in U.S. Pat. No. 4,486,296, Oleck, Dec. 4, 1984, that while zeolite catalysts have been employed in hydrocracking processes and although they may be effective in providing distillate yields having one or more properties consistent with the intended use of the distillate, these catalysts have come, in general, to suffer the disadvantage of not providing product yields having good low temperature fluidity characteristics, especially reduced pour point and viscosity.
The prior art has utilized a separate dewaxing process in order to reduce the pour point of the middle distillate using such selective intermediate pore size zeolites as ZSM-5 (U.S. Pat. No. RE. 28,398), and ZSM-23 (European Patent Application No. 0092376).
Other methods in the art to produce middle distillates possessing acceptable viscosity and pour point properties include processes wherein the hydrocarbon feeds are concurrently or sequentially subjected to hydrocracking and dewaxing in a continuous process using a large pore size zeolite hydrocarbon cracking catalyst such as zeolite X or zeolite Y and intermediate pore zeolite dewaxing zeolite catalyst, such as ZSM-5 (U.S. Pat. No. 3,758,402).
These processes have two drawbacks. The first is that while the pour point is reduced, the viscosity is increased, possibly above acceptable limits. Secondly, the process operates by cracking wax primarily to light products (e.g., C.sub.3 -C.sub.4) thereby significantly reducing distillate yield. International Application No. WO86/03694 published July 3, 1986 under the PCT discloses a hydrocracking process to produce high octane gasoline using a catalyst comprising silicoaluminophosphates alone or in combination with traditional hydrocracking catalysts such as zeolite aluminosilicates.
As noted above, the prior art processes for preparing middle distillates in high yields having appropriate viscosity and pour points involve the use of multisteps and/or multicatalysts.
It has now been discovered that middle distillate products can be selectively produced in a simplified process and over a single catalyst, in high yields and having reduced pour points and viscosities as compared to prior art processes.
It has now been found that heavy hydrocarbon oils may be simultaneously hydrocracked and hydrodewaxed to produce a midbarrel liquid product of improved yields and having satisfactory pour point and viscosity. This desirable result is obtained by the use of a catalyst which contains a SAPO-11 or SAPO-41 silicoaluminophosphate molecular sieve component, and which includes a platinum or palladium hydrogenation component to promote isomerization. Other conventional type metals, for example, nickel, tungsten, cobalt, molybdenum or combinations of these metals with platinum or palladium, e.g., Co-Mo, Ni-W, may also be used.